The present invention relates, in general, to photovoltaic devices, and more particularly to a photovoltaic device of a type having a front side exposed to a radiation from a radiation source and an opposite backside for converting radiation energy into electric energy.
Photovoltaic electric power generators are normally fixed and aligned to the direction of incident primary sunlight. Occasionally, the systems have also been equipped with a uniaxial or biaxial solar tracking unit, or sunlight concentrators have been used.
In particular when using light concentrators, there is the problem that the efficiency deteriorates at elevated temperatures of the photovoltaic device. The reason for that is the fact that the electrons, liberated by incident light photons, are partially thermally recombined so that the useful outer current flow of the photovoltaic module is reduced. The prior art attempted to solve this problem by providing on the backside of the photovoltaic modules, just like in electric components, heat-conducting sheets to improve the heat emission. In the event of higher temperatures, the photovoltaic modules are actively cooled by conducting a coolant across the backside of the modules. The processes for active as well as passive heat dissipation are, however, structurally complex and thus only rarely applied.
It would therefore be desirable and advantageous to provide an improved photovoltaic device obviating prior art shortcomings and exhibiting a higher efficiency.
According to one aspect, the present invention provides for a photovoltaic device for converting radiation energy into electric power which photovoltaic device includes a front side exposed to radiation from a radiation source, an opposite backside, and a cooling unit which includes a liquid medium arranged between the front side and the radiation source.
In order to prevent a reduction of radiation incident on the photovoltaic device, to date only cooling units have been proposed which are arranged on the backside of the photovoltaic device. The invention is based on the recognition that a cooling unit implemented with a liquid medium can also be arranged on the front side of the photovoltaic device. The liquid medium can hereby be so selected that the useful range of the solar spectrum for the photovoltaic effects is not, or only insignificantly, absorbed by the liquid medium, while the radiation energy in the regions which are of secondary importance for the photovoltaic effects are absorbed by the liquid medium. The liquid medium thus permits passage of radiation energy useful for photovoltaic effects and absorbs the remaining radiation energy.
It has been shown that liquids comprised substantially of water are particularly useful as liquid medium. Depending on the employed photovoltaic module, it is, however, also possible to use oils, alcohols or similar substances. These media can be further mixed with substances which optimize the filter characteristic in a solution or suspension.
A simple photovoltaic device can be realized by circulating the liquid medium between the front side and the radiation source as a result of differences in gravity between warm and cold portions. This configuration is known as thermosiphon and includes a process water reservoir having a lower region with a cold water outlet. From here, cold water flows into the lower region of the photovoltaic device and rises within the photovoltaic device to its upper end from where the water flows back again to the reservoir. As warm water enters the reservoir at a higher location, a temperature gradient forms in the water reservoir, with cold water at the bottom and warmer water in the upper zone. Heated water can be withdrawn directly from the reservoir. Preferred however is the disposition of a process water heat exchanger in the reservoir to heat cold process water to the desired process water temperature.
According to another feature of the present invention, the cooling unit may include a pump for circulating the liquid medium. This permits a flow of the liquid medium through the cooling unit to thereby continuously dissipate heat. The cooling unit may also include a thermostat for controlling the pump. This enables a combination of a sufficient cooling with an effective warm water recovery. The temperature adjusted at the thermostat and the pump output are determined by the required warm water temperature and the demanded cooling action.
Particularly good cooling actions can be realized when conducting the liquid medium directly across photovoltaic elements. The efficiency can be increased when conducting the liquid medium first across the backside and then across the front side of the photovoltaic device. The still cold medium is thereby heated on the backside of the device and absorbs further heat energy on the front side of the device. In this way, an effective cooling of the photovoltaic device is realized on the one hand, and a liquid medium at relatively high temperature is made available for further utilization. The efficiency can still further be increased by providing several cooling units arranged in parallel or serial relationship. Suitably, a further cooling unit is positioned at a distance to the front side. While this distanced cooling unit serves mainly as selective filter, a direct placement of the cooling unit on the photovoltaic modules allows at the same time filter effects and a cooling of the modules.
Particular filter characteristics can be realized with the selection of the cooling medium. According to another feature of the present invention, a selective radiation-transparent layer is disposed between the liquid medium and the radiation source. On the one hand, this selective radiation-transparent layer serves the conduction of the fluid, and, on the other hand, the combination of radiation-transparent layer and liquid medium produces a filter characteristic which is suited to the solar spectrum to be exploited for photovoltaic effects.
According to another feature of the present invention, the radiation-transparent layer may be coated in selected areas in radiation-transparent manner on the side facing the radiation source. The selection of different coating materials and processes, too, has an impact on the filter characteristic to attain an optimal characteristic filter curve in a positive manner.
Extensive series of tests have shown that a plate or a film of a fluoropolymer as radiation-transparent layer yields particularly good results. Especially, films of fluoropolymer are good for manufacture and useful for conduction of liquid coolant and as radiation filter. Good results have also been realized with acryl, polycarbonate and glass because these materials offer a high transparency in the incident spectrum and are mechanically stable as well as weather-resistant and waterproof. This can be implemented in a cost-efficient manner, for example, with double webbed acryl (PMMA) plates and double webbed polycarbonate plates.
According to another feature of the present invention, the radiation-transparent layer forms an envelope which surrounds the liquid medium. This envelope thus represents a closed component which is usable as filter and can easily be exchanged.
Persons skilled in the art will understand that the various configurations of a photovoltaic device according to the present invention are applicable for non-concentrated as well as for concentrated radiation.